vxlan: release lock after each bucket in vxlan_cleanup

We're seeing some softlockups from this function when there
are a lot fdb entries on a vxlan device. Taking the lock for
each bucket instead of the whole table is enough to fix that.

Signed-off-by: Sorin Dumitru <sdumitru@ixiacom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>